The invention relates to a multilayer film as a back cover for polyamide-containing solar modules with improved pigment and filler dispersion.
Solar modules, frequently also referred to as photovoltaic modules, serve for electrical power generation from sunlight and consist of a laminate which comprises a solar cell system as the core layer. This core layer is encased with encapsulation materials which serve as protection against mechanical and weathering-related influences.
In conventional solar modules, the active solar cell is positioned between a front cover and a back cover. The front cover is transparent, generally consists of glass; and is bonded by means of an adhesion promoter layer which often contains an ethylene-vinyl acetate copolymer to the layer comprising the solar cell. The back cover provides electric shielding, serves as protection against weathering influences such as UV light and acts as a moisture barrier.
Film composites composed of fluoropolymer films and polyester may conventionally be employed as a back cover. The fluoropolymer film on the outside provides weathering resistance, while the polyester film is utilized to obtain mechanical stability and electrical insulation properties. A further fluoropolymer film on the inside serves for attachment to the sealing layer of the solar cell system. However, such fluoropolymer films have only low adhesion to the sealing layer which is used as embedding material for the solar cells themselves. In addition, the fluoropolymer film contributes to electrical insulation only to a minor degree, which results in the need to use a comparatively thick polyester film.
WO 2008138022 therefore proposes replacing the two fluoropolymer films in such composites with films of nylon-12 (PA12). In a development thereof, WO 2011066595 proposes that the solar cell-facing thermoplastic layer comprise a light-reflecting filler such as titanium dioxide, while the solar cell-remote thermoplastic layer comprise a second filler such as glass fibres, wollastonite or mica, which brings about a higher thermal conductivity of this layer. Illustrative thermoplastics come from the group of the polyamides, polyesters or blends of polyamide and polyolefin. Explicit mention is made of PA11, PA12 and PA1010, and blends thereof with polyolefins.
In such photovoltaic modules, the distribution of the light-reflecting filler is of particular significance. Thus, the reflection level and hence the power yield of the solar cell increase with increasing dispersion. In addition, the quality of distribution simultaneously influences the surface quality of the backing film. This is exceptionally important in the generally purely polyamide-based layer of the back cover, said layer facing the actual solar cell and being visible on the outside, since pigment agglomerates become particularly apparent here due to the low layer thicknesses.